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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097 or send an email to silicon@siliconchip.com.au

Speed control for a 110V movie projector

Would either of the universal motor speed controller circuits published this year (February or May 2009) be suitable for running a 110V AC motor. Could they be easily adapted?

The application is a French Pathe 9.5mm film projector, circa 1950, that runs off 110V AC via a stepdown transformer. The motor speed control (not original) is currently via a series wirewound pot. (P. H., Warwick, Qld).

• The May 2009 motor speed control would be suitable. One of the 4.7kΩ 5W resistors supplying the 15V supply would need to be replaced with a wire link to provide the necessary current from 110VAC.

Power supply needed for Mighty Midget

I have recently built the Mighty Midget power amplifier published in March 2002 and I am having trouble getting it going. I have music going in but I am only getting half a watt out. The sound is distorted and crackly and it starts oscillating.

I have checked all solder joints and tested the power consumption (110-150mA) and all checks out OK. I am running it off my lab power supply at 13.8V with a 4-ohm loudspeaker.

I am pretty sure that the TDA1562Q chip was not heat-damaged by my soldering iron. Can you help? It could have some thing to do with the muting feature inside the TDA1562Q. (R. C., Menai NSW).

• The power supply needs to be hefty enough to prevent the voltage dropping under load when driving the speaker. It should work with a 12V car battery or with a power supply that has sufficient current capability (4A). Some 2200µF capacitors across the power supply will help.

Audio power
meter modifications

I am wondering if it is possible to modify the Appliance Power Meter (SILICON CHIP, July & August 2004) so it can measure audio power through a resistive load? (G. M., via email).

• It is possible. The voltage inputs connecting pins 6 & 7 of IC1 to the mains supply (via 2.2MΩ and 1kΩ resistor dividers) would need to be disconnected from the mains supply and connected to the amplifier output. Similarly, the current measurement resistors R1 and R2 (going to inputs at pins 4 and 5 of IC1) would need to be disconnected from the mains and connected so that the amplifier load current flows through the sense resistor, R1.

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